I would avoid August unless you absolutely had no choice. You will have the sweltering heat and some of the largest crowds outside of the holidays. December I would only choose in the early part of the month, and definitely avoid Christmas Week and the week before(highest crowds of the year). New Years Eve only if you wanted to be there for the celebration, as the crowds can get pretty big then as well. In December, I would go in the earlier part of the month to have a slower Christmas Party(if you are into that) or even the last couple weeks of November. To avoid the problems with August, you could go the last couple weeks of September or the first week or two of November and hit Food & Wine and
MNSSHP if you wanted. Just a couple of alternatives to consider. Of the three you listed, the first couple weeks of May would be my choice. Decent weather for the South(Florida isn't much different from my home state of Virginia), though it can be slightly warm(but not as bad as August), and it's before schools are out for summer so you don't have all the crowds. As for Covid, I wouldn't worry too much about that yet for 2021. Make your plans and have a contingency in mind in case you need to reschedule. I am currently trying to plan a trip to WDW, a trip to DLR, and a trip to Europe all around each other between now and the end of 2021, so you can imagine how much trouble I am dealing with.